A typical video signal comprises a number of frames, which when displayed at an appropriate speed, such as 25 frames per second, provide a viewer with an impression of viewing a continuous scene. A digital signal contains, in its raw form, a sequence of frames to be displayed. A number of techniques are known for encoding the raw digital signal, wherein most of the techniques are directed to decreasing the bandwidth necessary to transmit the signal while maintaining an acceptable quality level for given audience. When the encoded digital video signal is to be displayed, it is decoded into its raw form and consecutive frames are transmitted to the display in order to present them to the viewer at a given speed.
A stereoscopic video signal comprises alternate left and right frames, i.e. frames to be viewed by the left eye and the right eye of the viewer, to be displayed at a higher speed than a mono video signal, such as 60 frames per second. The signal may be viewed by so-called shutter glasses, such as LCD shutter glasses, the operation of which is synchronized with the display. When the left frame is displayed, the left shutter glass is open and the right shutter glass is closed, while when the right frame is displayed, the left shutter glass is closed and the right shutter glass is open. A basic stereoscopic system of this type is disclosed in the U.S. Pat. No. 4,424,529 “Remotely triggered portable stereoscopic viewer system”.
A typical video display, such as an LCD display, needs some amount of time, typically in the order of 5 ms to 8 ms, to redraw the currently displayed frame to display a next frame, called a frame redraw time. It is important to synchronize the operation of shutter glasses such that a shutter glass is open only when the frame is completely redrawn, so as to provide the viewer with a complete picture. Therefore, during redrawing of a frame, the left shutter glass and the right shutter glass are both closed. When the frames are displayed at a rate of 60 frames per second, the time allotted for a single frame for one of the eyes is 16.7 ms, wherein the frame is displayed for example for 11.7 ms and redrawn for 5 ms. In an ideal situation, when the shutter glasses are perfectly synchronized with the display, the active viewing coefficient, defining the percentage of time at which one eye of the viewer may watch the video signal, is 11.7 ms/2*16.7 ms=35%. The coefficient can be much worse for older types of displays having longer redraw time, such as 20% for a 10 ms redraw time. In practice, there may be an additional shutters closing time necessary to be introduced in order to compensate for various signal delays. Various attempts have been made to improve the synchronization, in order to arrive at a high active viewing coefficient while keeping proper synchronization, such as the method disclosed in the U.S. Pat. No. 6,678,091 “System and method to synchronize one or more shutters with a sequence of images”.
The lower the active viewing coefficient, the less information is received by the viewer's eyes and the worse impression of the viewed signal, due to worse brightness and color perception. The aim of the present invention is to improve the viewer impression when viewing a video signal, in particular a stereoscopic video signal. This aim is achieved by providing a video signal which can be displayed with a shorter frame redraw time. In case of mono video signals, this results in a more stable display. In case of stereoscopic video signals, this allows achieving higher active viewing coefficient when viewing the signal via shutter glasses.